During engine operation, combustion gas may leak between the cylinder and the corresponding piston rings, and into the engine crankcase. The leaked combustion gas is referred to as blowby gas, and typically includes unburned intake air, fuel, exhaust gas, oil mist, and water vapor. In an effort to ventilate the crankcase and re-circulate the blowby gas to the intake side of the engine, a positive crankcase ventilation (PCV) system is provided.
An air-oil separator is provided for separating the blowby gases from oil and mist. In a turbocharged engine, a portion of the blowby gas passes through the air-oil separator is then routed to the inlet of the turbocharger through a PCV line. The turbocharger is connected to an intake manifold of the engine. During high boost conditions, a partial vacuum may be created at the turbocharger inlet and inside the crankcase of the engine. The partial vacuum is created when airflow into the turbocharger increases, under high boost conditions. This in turn may cause the negative pressure limit of the crankcase to be exceeded. The crankcase includes lip seals that are typically mounted between the engine block and the crankshaft, and are used for sealing and to prevent the ingress of contaminants and oil leakage. However, exceeding the negative pressure limit of the crankcase may cause the lip seals to be pulled out of their seated positions.
In one approach, the level of crankcase vacuum is limited by reducing a portion of the PCV vent line diameter. In an alternative approach, an orifice with a specified size is provided in the PCV vent line. However, selecting an appropriate PCV line diameter or orifice can often take a significant amount of time, testing and development to obtain the desired crankcase pressures under high boost conditions. Moreover, even when the appropriate PCV line diameter or orifice is selected, PCV icing or crankcase NOx requirements may not be met. Specifically, in cold climates condensed water in the PCV system can gather and freeze, especially in areas where the PCV line diameter has been reduced. Frozen water in the PCV line can lead to an obstructed gas flow, or to icing of the vent system components. NOx requirements are adversely affected because a restricted gas flow in the PCV line will in turn lead to less fresh air being supplied to the engine crankcase.
In yet another approach to limit the level of crankcase vacuum, the PCV line length is significantly extended. However, this approach will also have the tendency to gather condensed water in the PCV line and freeze. Accordingly, it is desirable to provide a PCV system that will not exceed a specified crankcase vacuum at high boost conditions while still meeting PCV icing and crankcase NOx requirements.